Organic Chemistry of Explosives

Nitration 131

heating or cooling. Temperature increase during nitration is often associated with an increase

in undesirable by-products (Section 4.3.5). However, many deactivated substrates, particularly

those already containing one or more nitro groups, react only slowly with mixed acid at ambient

temperature.

While nitric acid is generally a good solvent for many organic compounds, the mixed

acids commonly used in nitrations, especially those containing oleum, result in heterogeneous

reactions, meaning that a high stir rate is important for uniform substrate nitration. When

working with mole quantities of substrate a high stir rate also becomes an important safety

point because heterogeneous nitrations containing large portions of unreacted substrate are

susceptible to thermal runaway and the risk of explosion.

Experimentally, the use of mixed acid for the nitration of aromatic substrates is very con-

venient. Reactions are often quenched by the addition of water, where the product usually

precipitates. Solids are simply filtered from the acid liquors and oils are either separated or

extracted into organic solvents. However, on an industrial scale, these mixed acid nitrations

create environmental problems from air and water pollution (Sections 4.3.5 and 4.8.2).

4.3.2 Substrate derived reactivity

4.3.2.1 Phenols and phenol ethers

Phenols and phenol ethers are nitrated with relative ease, and even though the deactivating

effect of the nitro group means that progressive nitration becomes slower and more difficult,

the introduction of three nitro groups into the aromatic ring can be achieved under fairly mild

conditions. However, the direct nitration of substrates containing phenolic groups can be low

yielding because of facile oxidation and the vigour of such reactions. Phenol itself is readily

oxidized to oxalic acid on heating with concentrated nitric acid. The direct nitration of phenol

with mixed acid provides low yields of picric acid along with much resinous matter, the acidic

liquors also containing oxalic acid. With all phenolic substrates, the higher the temperature of

the nitration, the more by-products formed.

OH

SO 3 H

OH OH

NO 2

NO 2

4

O 2 N

OH

SO 3 H

SO 3 H

20

SO 3 H

21

OH

18

1. 98 % H 2 SO 4 , 100 °C
   2. 70 % HNO 3
   66 % (2 steps)

19

Figure 4.5

A commonly used strategy for the higher nitration of phenolic substrates is to sulfonate the

electron-rich aromatic ring before nitration. Sulfonic acid groups are electron withdrawing and